1. Field of the Invention
The present invention relates to a semiconductor device used as a diode or a transistor, and, more particularly, to a semiconductor device of a vertical type which is used at a high voltage.
2. Description of the Related Art
A semiconductor device with the structure as shown in FIG. 4 has been known as that used for diodes and the like. Such a semiconductor device comprises a p-n junction 32 formed by implanting or diffusing p-type ions into an n-type semiconductor layer 31 with relatively high resistance, a first electrode 34 provided on one surface of the n-type semiconductor layer 31, a protective insulation film 35 provided on the other surface, and a second electrode 36 for connection to the p-n junction 32 provided on the protective insulating film 35.
In the semiconductor device shown in FIG. 4, when the n-type semiconductor layer 31 consists of a compound semiconductor such as a GaAs semiconductor, surface recombination occurs due to a high surface level so that the performance as a semiconductor is significantly deteriorated.
Then, a semiconductor device shown in FIG. 5 is known which is an improvement on that shown in FIG. 4. This semiconductor device comprises a surface-inactivated semiconductor layer 41 provided on the surface of the n-type semiconductor layer 31 opposite the first electrode 34, the protective insulation film 35 provided on the surface-inactivated semiconductor layer 41, and a second electrode 36 for connecting to the p-n junction 32 provided on the protective insulation film 35.
In the semiconductor device shown in FIG. 5, however, breakdown phenomena (dielectric breakdown) such as avalanche breakdown or zener breakdown, and, more particularly, avalanche breakdown, tend to occur when a high voltage is applied to the electrodes 34 and 36. Avalanche breakdown is divided into surface breakdown and bulk breakdown. Surface breakdown is breakdown occurring near the device surface due to the concentration of electrical fields caused by ions or levels existing on the device surface or interface, and occurs at a lower voltage than that for bulk breakdown.
For example, in the semiconductor device shown in FIG. 5, when a voltage is applied across the electrodes 34 and 36, there is a tendency for the concentration of electrical fields to occur at a part of the n-type semiconductor layer 31 immediately below the end 36-1 of the electrode 36. Thus, it is believed that, if there is a p-n junction 32 near the end 36-1 of the electrode 36, breakdown is caused at the p-n function 32 by the above-mentioned concentration of electrical fields.
Then, the inventors worked to provide the field plate structure 36-2 shown by phantom lines in FIG. 5 by extending the end of the electrode 36 to relax the effects of the concentration of electrical fields. The field plate structure itself is known as described in Japanese Laid-Open Patent Application No. H1-136366. In this Connection, the invention of that application applies a field plate structure to the base electrode of an FET with a high-resistance semiconductor layer consisting of monocrystal Si, and constructs the underlying insulation layer of the base electrode in a multilayer structure to provide a step.
It is believed that the field plate structure 36-2 relaxes the concentration of electrical fields near the p-n Junction 32, and that a semiconductor device with good dielectric breakdown performance can be obtained. However, experiments conducted by the inventors revealed that, even if the length 1 of the field plate structure 36-2 were extended to about 50 .mu.m as shown in FIG. 6, the device could only withstand a voltage less than 650 V, and that a semiconductor device capable of withstanding a voltage of 700 V or more could not be obtained.
Furthermore, the inventors worked to improve dielectric breakdown performance by thickening the protective insulation film 35 under the electrode 36 so that the concentration of electrical fields was divided and borne by the protective insulation film 35 and the n-type semiconductor layer 31. However, the study by the inventors revealed that, if the thickness of the protective insulation film 35 was increased to 5000 82 m more as shown in FIG. 7, contrary to the expectation, the dielectric strength tended to be deteriorated.